The retina is the innermost layer of the wall of the eyeball located in the posterior segment of the eye. Developed as an outgrowth from the brain, the retina contains nervous tissue, specifically, light-sensitive cells (photoreceptors) and complex neural networks. These networks provide visual information and send impulses through the optic nerve to the brain.
Degenerative diseases of the retina, such as retinitis pigmentosa, age-related macular degeneration, and hereditary retinal degenerations cause degeneration and death of the photoreceptor cells, resulting in decreased visual function. Fortunately, even in end-stage disease, numerous neuronal cells in the inner retina survive. However, because of loss of the photoreceptors, light stimulation does not occur and the neuronal cells must be artificially stimulated to restore some degree of visual function.
The neuronal cells may be stimulated, either from the outer surface of the retina or from the so-called subretinal space. One technique for subretinal stimulation uses semiconductor microphotodiode arrays (SMA) as described in Peyman et al., Ophthalmic Surgery and Lasers, 1998, vol. 29, p. 234, which is expressly incorporated by reference herein in its entirety. These arrays are fabricated by standard photomask and etch techniques, and can be produced with thicknesses ranging from about 10-200 μm and sizes varying from about 0.5-5 mm in diameter. The arrays are separated into subunits, which create a pixel density of over 1000 subunits/mm2. The subunits have no electrical connection; they are powered by incident light having a wavelength between 500-1100 mm. Another technique for retinal stimulation uses an electrode array to electrically stimulate the neurofiber layer of the retina. The array has 25 platinum disks arranged in a 5×5 square, as reported by Majji et al., Investigative Ophthalmology and Visual Science, 1999, vol. 40, p. 2073. The improved surface of the platinum disk forms a planar array of stimulating electrodes in a silicon matrix that is less than 1 mm thick. Twenty-five wires originating from the disk form a cable which extends from the array and is at least 10 cm long and 600 mm thick. To implant the array into the eye, the surface of the implant (3×5 mm) is placed over and is fixed to the retina either by mechanical fasteners such as pins or tacks, or by bioadhesives.
There are, however, several disadvantages of implanting the aforementioned types of arrays into the subretinal space. One disadvantage is that the implant may interfere with nutrition of the retina, since nutrients come partially from the choroid (the back of the retina). Fenestrations, or small openings in the array, can help to maintain nutrient accessibility to the retina. Implanting the electrode type of array over the surface of the retina has additional drawbacks. One drawback is that fixing the array over the retina is very difficult. If pins or other mechanical fasteners are used, they should penetrate the entire retina and reach the scleral wall in order to secure the array, but this increases the risk of hemorrhage from the retinal and choroidal circulation. The increased fibrous proliferation around both the fasteners and the array also causes localized scarring and traction on the retina. Another disadvantage is that electrical stimulation in the subretinal space may not adequately excite the ganglion cells and the neurofiber layer, which are located in the outer portion of the retina.
Drugs such as gancyclovir or various steroids can also be administered to the patient to attempt to prevent, halt, or alleviate the pathological process. Ocular drugs may be administered systemically, parenterally, or topically. Alternatively or additionally, the drugs may be administered in a slow release formulation.
While current methods exist for treating patients experiencing a loss in visual function due to retinal pathology, several problems still remain. Thus, additional methods to improve visual function, while decreasing or eliminating these problems, are desirable.